Evaluation of Interior PM and Surface PM Synchronous machines with distributed and concentrated windings

This paper presents a detailed evaluation and comparison of surface PM and interior PM synchronous machines with concentrated and distributed windings. The machines are compared based on optimal designs having the same stator OD, stack length, identical magnet volume, equal air-gap length and same peak current. The maximum open circuit voltage is also included in the trade-off analysis and machines with constrained and unconstrained maximum BEMF are considered. The study shows that for designs with equal maximum BEMF the IPM motor exhibits a much higher peak torque density than the SPM. This is because in an IPM motor the reluctance torque more than offsets the reduction in permanent magnet torque due to the reduction in q-axis current. The study also shows that, for the given set of design constraints, even if the maximum BEMF is not limited, the SPM outperforms the IPM by less than 2%. The use of concentrated windings yields a slightly higher torque density for very short stack length designs but a longer stack increases the weight.